It's a complicated situation but basically they got lucky. King Harold lost a lot of his best men against the Vikings and he still almost beat the Normans. If it was just the Saxon's vs the Normans the Saxons would have won easy. After the Battle of Hastings the Saxons also didn't unite properly partly because the two battles had killed so much of the ruling class. Also the Normans had Scandinavian blood maybe that made them superior to ordinary French.